The aggregate-forming, nonheterocystous, filamentous blue-green alga (cyanobacteria) Trichodesmium spp. is a widespread and important planktonic N2 fixer and primary producer in tropical and subtropical oceans. It is unique among nonheterocystous genera because it conducts N2 and CO2 fixation (O2 evolution) simultaneously; a notable achievement, because O2 is a potent inhibitor of N2 fixation. Spatial and temporal CO2 fixation patterns were examined in trichomes and aggregates from natural and cultured populations, utilizing microautoradiographic detection of 14CO2 incorporation. Parallel N2 fixation (acetylene reduction) measurements were also made. Diel N2 and CO2 fixation patterns were similar, with co-optimization of both processes near midday. Microautoradiographs revealed several trichome-level 14CO2 incorporation patterns: 1)uniform, heavy labeling, 2)uniform, light labeling, 3) heavier labeling in distal as opposed, to proximal regions, and 4) virtually no labeling throughout. Similar patterns were observed in natural and cultured populations. Given previous immunochemical findings that N2 fixation potential is widespread in Trichodesmium spp. trichomes and aggregates, current results suggest a high degree of individuality, and possibly a “division of labor” in terms of CO2 fixation, among trichomes comprising active N2-fixing aggregates. Segregation of photosynthesis within and among trichomes facilitates simultaneous N2 and CO2 fixation in Trichodesmium spp. trichomes and aggregates.